The present invention relates to an automotive engine having a turbocharger, and more particularly to a system for controlling the flow of intake air for the turbocharged engine.
In a conventional turbocharged engine, a turbocharger comprising a compressor and a turbine is located between an intake manifold and an exhaust manifold. The intake air is necessarily passed through the compressor, regardless of driving conditions of a motor vehicle. The compressed air is supplied to cylinders of the engine and the flow of the compressed air is regulated by a throttle valve. In order to control compressed air in the intake system for preventing the engine from overcharging, a wastegate is provided in a bypass around the turbine.
When the throttle valve is partially opened at a partial load of the engine, the intake air supplied by the compressor is forcibly throttled. Accordingly, the pressure of the intake air between the compressor and the throttle valve is increased, so that load on an impeller of the compressor increases to reduce the rotating speed of the impeller, that is the turbine. As a result, the back pressure of the exhaust gas in an exhaust system increases. Accordingly, pumping loss increases which causes the fuel consumption to deteriorate.
When the throttle valve is fully opened, the pumping loss does not occur. However, the speed of the impeller does not increase in proportion to the increase of the opening degree of the throttle valve. Consequently, the turbocharging for the wide throttle open delays. Accordingly, in the turbocharged engine, it is desired to reduce both the pumping loss and the time lag.
Japanese Patent Application Laid-Open No. 55-72627 discloses a control system in which a bypass passage having a control valve is provided in a bypass around a compressor and a wastegate is provided in the bypass for the turbine, for controlling the flow of the intake air and the exhaust gas. The control valves are operated to be closed at acceleration of the engine.
In such a system, the pumping loss is educed at a partial opening of a throttle valve, since both of the bypasses are opened. However, at the partial load, the turbine does not operate, because exhaust gas is not applied to the turbine. Accordingly, when the throttle valve is widely opened for accelerating the engine, the turbocharging effect is greatly delayed.